Cassini Sees Debris from Meteoroids Colliding with Saturn’s Rings

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!

The bigger a target you are, the more at risk you are, of course, and things don’t come much bigger than Saturn’s rings. At 300,000+ kilometers across (180,000 miles), they present a juicy target for meteoroids—heck, from above they even look like a bulls-eye!

The image above shows four of the collisions seen by Cassini. The one at the top left was caught about a day after impact, and the picture to the right of it shows the same impact about a day later. The other pictures show different impacts caught mere hours after they occurred.

Saturn’s rings are not solid, but are composed of countless trillions of small icy particles. When a meteoroid (a small solid piece of errant material) plows through them at high speed, it vaporizes and creates an expanding debris cloud. The astronomers doing the study think that this slows the particles down, putting them in orbit around Saturn. Later, these secondary particles collide with the rings at slower speeds, causing the long trails of debris seen here.

You’d think that would be easy to spot, but in reality these features are pretty difficult to find. The event has to be big enough to see, first of all. The rings are peppered with extremely small impacts all the time, but from hundreds of thousands of kilometers away Cassini doesn’t have much chance to see them. Bigger ones are more rare, which is why so few have been seen.

Plus, the lighting conditions have to be right. In the first four images above, the rings were nearly edge-on to the Sun (it was Saturn’s spring equinox), so the trailed clouds of impact debris were easier to spot rising out of the plane of the ultra-thin rings—to scale, the rings are thinner than a sheet of paper!

The meteoroids that hit the rings range in size from a centimeter to several meters in size. Interestingly, the astronomers who did the new study were able to estimate the rate of collisions judging from the impacts seen, and apparently for objects this size they happen at Saturn at about the same rate they do here at Earth. Saturn is in a relatively emptier region of space than Earth is, but its gravity is also far higher. These two factors must more-or-less cancel, so that impact rates are about the same there as here.

Animation showing the expansion and shearing of a debris cloud in Saturn's rings. Click to impactenate. Animation by NASA/Cornell

Another interesting fact is that the clouds expand and shear out over time as the particles from the collision orbit Saturn at different rates. This allows astronomers a way to determine how long it’s been since the impact event when looking at a picture.

A clump of material bursts through Saturn's faint F-ring, seen in 2009 by Cassini. Click to embiggen.

Features similar to this have been seen before, but those were low speed collisions caused by material in the ring itself clumping up and moving around, disturbed by the gravity of a passing moon. While dramatic and beautiful, those “mini-jets” are a far less energetic phenomenon than what’s seen here.

A side note: This result was only made possible because Cassini was designed to go to Saturn and stay there, exploring that alien environment for years. The longer we look, the more we learn about the weird goings-on around Saturn. It’s a locale of forbidding beauty and incredible vistas, and it teaches us a lot about the solar system, about planets, and about the way the two interact. This knowledge at the very least teaches us more about the Earth, and is useful… but I prefer to think about ding this as learning for the sake of learning. It’s what humans do, and I think it’s in large part what makes us human. We’re curious. We explore. We learn.